Atlantic Tropical Activity To Heat Up As El Niño Fades

El Niño conditions continue this summer but not for much longer. The June Niño 3.4 index (the primary ENSO measurement) was 0.6°C above the long-term average (El Niño is present when the average sea-surface temperatures across the central & eastern tropical Pacific are 0.5°C or greater over an extended period of time). The Climate Prediction Center is forecasting ENSO-neutral conditions to resume within the next month or two as the tropical Pacific cools (see Fig. 1). Over the past month, there has been a gradual decrease in equatorial Pacific sea-surface temperature anomalies (see Fig. 2). What are ENSO-neutral conditions? ENSO-neutral conditions are when neither El Niño or La Niña conditions are present.

Fig. 1: Histogram showing probabilities for El Niño (red), neutral (gray) and La Niña (blue) conditions for the remainder of 2019 into 2020 (map by Climate Prediction Center)–**while there is a slight decrease for neutral conditions from fall into winter, neutral conditions is the most likely scenario
Fig. 2: Change in sea-surface temperature anomalies since June 12th (map by Climate Prediction Center)

The state of ENSO is very important in the evolution of tropical activity across both the Atlantic and Pacific Basins. El Niño acts to suppress tropical activity in the Atlantic Basin. The reason for the suppressing of tropical activity in the Atlantic Basin is due to enhanced vertical wind shear (see Fig. 3). What is vertical wind shear? This is the changing of wind speed and direction from around 5,000 feet to 35,000 feet above the ground. Wind shear is not good for tropical cyclogenesis because the shear can act to rip apart a developing hurricane and/or prevent a tropical wave from developing any further.

Fig. 3: Typical influence of El Niño on Pacific and Atlantic seasonal hurricane activity (map by NOAA–based on originals by Gerry Bell)

With El Niño expected to diminish within the next month or two, it is possible the Atlantic Basin will become active if the atmosphere responds in time to the ENSO-neutral conditions. Climatologically, ENSO-neutral and La Niña tropical seasons are more active than El Niño season (see Fig. 4).

Fig. 4: Atlantic hurricane activity during El Niño, ENSO-neutral and La Niña season (map courtesy the Weather Channel)

Keep in mind, hurricane season runs from June 1st through November 30th–leaving plenty of time for tropical activity to become enhanced. It should be noted, the peak of hurricane season is August through September in which El Niño conditions will likely continue through the climatologically busiest period. It looks like the hurricane season will become more active as we head into a more climatologically active period paired with the development of ENSO-neutral conditions. Other factors can and do influence tropical activity such as the strength of the west African monsoon and the sea-surface temperatures across the Atlantic Basin.